Mark Sobell is a class act. He has been a mitzvah to both the programming and writing sides of the *nix equation for 30 years. Free is about $50 less than this download should cost.
I find that, ironically, professors are far too cavalier with posting other people's content online without permission.
The only real basis for my opinion is seeing .edu somewhere in the URL when these things are posted. So, I could be wrong, but I can remember two other recent examples: an HBS case, and a couple of chapters from Jared Dimond's Guns, Germs and Steel.
